So-called "hot melt" adhesives are used for various coating and bonding operations such as diaper construction, package forming, automobile parts assembly and electronics coating. Generally, it is convenient to store and ship the hot melt adhesive materials in bulk forms such as chicklets, slats, pellets and bricks and also in slugs held in containers, such as relatively small one or five gallon pails, or in containers as large as 55 gallon drums depending on the quantity necessary for the application.
Hopper-type dispensers may be used to melt and dispense many forms of hot melt adhesives including those mentioned above. In the case of bulk adhesive in the form of a slug held in a container, hopper-type dispensers are used to first remove the slug from the container and then to melt the slug of adhesive as it contacts a heated melting grid mounted near the bottom of the hopper. Other forms of adhesive may simply be loaded into the hopper. A heated reservoir is usually disposed beneath the melting grid and receives the melted or liquified hot melt adhesive as it passes through the grid and then maintains the adhesive in a heated liquid state suitable for the application. An outlet of the reservoir typically leads to a pump and manifold assembly for pumping the hot melt adhesive to a dispenser appropriate for the application.
The melting grids in hopper type melter units are the primary devices for transforming the adhesive from its initial form into a molten or at least semi-molten state. Melting grids may consist of various forms of heated members which melt the adhesive on contact. The grid also includes through passages for the melted adhesive. To maximize the melting rate, it is generally desirable to increase the number of heated grid members to create maximum heated surface contact with the end of the adhesive slug. However, the through passages between the grid members must be large enough to prevent back up of melted adhesive above the grid and to allow sufficient flow of melted adhesive into the reservoir to maintain the supply of liquid adhesive to the dispenser. Finally, the grid must also be strong enough to support the weight of the adhesive which may be on the order of several hundred pounds in the case of a 55 gallon drum of adhesive.
On account of the competing desirable grid characteristics noted above, certain problems and trade-offs must be addressed when designing melting grids. For example, if the individual grid members are too numerous, too wide and/or too closely spaced, then there may be a high melt rate, however, the through passages between the grid members will be too small and the flow rate through the grid too low. On the other hand, if there are not enough grid members, or the grid members are too narrow or too widely spaced, then there may not be enough surface area contact between the grid members and the adhesive in the hopper to achieve a necessary or desirable melt rate. In addition, in this latter case the grid might be too weak to repeatedly support the weight of heavy adhesive slugs over an acceptable length of time or "useful life".
In general, therefore, a need has existed for a melting grid which produces relatively high melt rates and flow rates and which is further able to support the weight of the adhesive in its initial form over a long useful life.